Known metal-adsorbing materials include; a materials such that metal-ion-adsorbing functional groups are chemically bound on a carrier by which they are immobilized, materials such that a low-molecular compound containing one or more of such functional groups and enclosed in a carrier to prevent its dissolution, and the like. These metal-adsorbing functional groups can be carboxyl groups, sulfonic groups, amino groups, thiol groups, phosphoric groups, or the like. The readily-adsorbable metal species differs depending on the kind of the metal-adsorbing functional groups, and sulfonic groups primarily adsorb monovalent metal ions such as potassium or sodium ions while carboxyl groups and phosphoric groups adsorb all metals including calcium and magnesium. Amino groups, imino groups and thiol groups are considered to show strong adsorption for heavy metals. Accordingly, the kind of metal-adsorbing functional groups to be used is selected depending on the metal species to be adsorbed.
A wide variety of carriers are also used, including vinyl resins such as styrene resin and acrylic resin, and as natural materials, cellulose (powder, fibers, gel), chitin, chitosan, wool and the like are in use. In the production of a metal-adsorbing material, the production is, in many instances, conducted subsequent to the introduction of one or more metal-adsorbing functional groups in a polymerizable monomer from the standpoint of ease in synthesis and uniformity in quality. Such metal-adsorbing materials are known as ion exchange resins or chelate resins. Chemical introduction of metal-adsorbing functional groups into natural materials such as cellulose or wood is also performed as those raw materials and also production costs are inexpensive.
The metal-adsorbing capacity of a metal-adsorbing material is also significantly affected by the shape of its raw material in addition to the chemical structure of the raw material. A resin is often formed into beads in view of its properties; however, it involves problems on the efficiency of treatment in that the content of the functional groups enclosed within the beads will be larger; the speed of diffusion of metal ions or a regenerant into the resin becomes low due to the hydrophobicity of the resin; and the lowest adsorbable concentration becomes high.
The manner of use of a metal-adsorbing material is also limited by the shape of its raw material. Upon treatment of water for the removal of a metal therefrom, a metal-adsorbing material with metal-adsorbing functional groups graft-polymerized on a resin-based raw material or a raw material cut into short pieces is limited to a method that the metal-adsorbing material is added into the water layer and subsequent to adsorption of the metal, the metal-adsorbing material is recovered by centrifugation or filtration; or to its use in a column. When the raw material has a small particle size, the filtration rate becomes low; therefore, such a raw material is not suited for the treatment of a great deal of water.
In the case of treatment for the removal or collection of a metal, especially the softening of water, the removal of a harmful metal from industrial wastewater, the removal of a harmful metal or the collection of a valuable metal from contaminated soil, or the like, the scale of an amount or area to be treated is enormous so that a large quantity of metal-adsorbing material will be necessary. The metal-adsorbing material to be used for these purposes is, therefore, desired to be high in adsorbing capacity; low in price; and reusable.
As metal-adsorbing functional groups, phosphoric groups have merits in that 1) one divalent metal ion can be adsorbed with a single phosphoric group and hence, more metals can be adsorbed, 2) they tend to liberate hydrogen ions on an acidic side and thus, the pH range of solutions from which metals can be adsorbed is broad, and 3) the lowest adsorpable metal ion concentration is low. As a carrier, on the other hand, cellulose has merits in that 1) the fibers themselves are highly durable, 2) its functional groups are mostly exposed on fiber surfaces, and 3) it has high formability.
For these merits, metal-adsorbing materials formed of natural materials as carriers containing phosphate ester groups have drawn interests. Known research includes, for example, use of cellulose phosphate for the removal of heavy metals and radioactive metals (Patent Document 1), a method for enhancing the mechanical strength of fibers by using sulfur powder upon production of cellulose phosphate (Patent Document 2), use of cellulose or starch phosphate, acetate or benzoate for the removal of heavy metals from water (Patent Document 3), and use of a filter, which is composed of cellulose having carbamido groups and phosphate ester groups, for the removal of hardening components or heavy metals from drinking water (Patent Document 4).
They are, however, not sufficient from the standpoint of metal-adsorbing capacity or economy, leading to an outstanding strong desire for the development of an economical metal-adsorbing material with still better metal-adsorbing capacity, metal-reacting velocity, mechanical strength, formability, application range, reusability and so on.
Cellulose exists in four polymorphs ranging from polymorph I, which is a natural cellulose, to polymorph IV. Cellulose I phosphate available from the phosphorylation of natural cellulose is known as described above, but the phosphorylation product of cellulose II is not known.    Patent Document 1: RU-C1-2096082    Patent Document 2: WO-A1-99028372    Patent Document 3: DE-A1-19859746    Patent Document 4: JP-A1-2003-500199